There are many medical and cosmetic treatments that utilize electro-magnetic radiation to destroy a local defect in skin. Among these are laser-assisted hair removal, vascular lesion treatment and skin rejuvenation. In these treatments, the defect is irradiated, and heat formed in the skin in and near the defect, destroys the defect. The main problem limiting broad use of these treatments is the risk of thermal damage to skin surrounding the defect. One way to reduce this risk is to monitor skin temperature during the irradiation, and to stop the irradiation before the skin becomes overheated. However, surface skin temperature measurements give information relating only to the superficial skin layer, composed of dead cells (stratum corneum), while the temperature of the underlying living tissue can differ significantly.
One method for assessing tissue temperature relies on measurement of the electrical impedance of the tissue, which is temperature dependent. Tissue impedance decreases 1% to 3% for every centigrade degree increase in. temperature (Francis A. Duck, Physical properties of tissue, a Comprehensive Reference Book, Academic Press, 1990, p. 173), U.S. Pat. No. 5,643,257, discloses a method for invasive thermal treatment of varicose veins, in which irradiation intensity is reduced when the tissue impedance drops below a predetermined value.